Field of the Invention
The present invention concerns a method of checking the functional capability of a catalytic converter.
In lambda-controlled internal-combustion engines, a catalytic converter, usually a three-way catalytic converter, is used in the exhaust system for emission control, wherein the efficiency of the catalytic converter is essential for the quality of the emission control. However, aging and contamination reduce the converting capability of the catalytic converter and consequently its efficiency. It is known that the oxygen storage capability of a catalytic converter decreases with its efficiency. The oxygen storage capability of a three-way catalytic converter is, however, strongly temperature-dependent.
To ensure that given limit values are met during the operation of the internal-combustion engine, the efficiency of the catalytic converter is checked on the basis of the oxygen storage capability. If the efficiency of the catalytic converter falls below a predetermined threshold, which may for example have the consequence that the exhaust gas emitted does not meet legal regulations, a signal indicating a defective catalytic converter is generated.
Published Non-Prosecuted European Patent Application EP 0 444 783 A1 discloses a method of monitoring the efficiency of a catalytic converter in which an oxygen sensor which measures the oxygen concentration in the exhaust gas is provided at the outlet of the catalytic converter. Since in conventional lambda control the air/fuel ratio fed to the internal-combustion engine is set such that it oscillates about the lambda value one, the oxygen sensor downstream of the catalytic converter measures an oscillating oxygen concentration. According to Published Non-Prosecuted European Patent Application EP 0 444 783 A1, the difference between the maximum value and the minimum value of this oscillating oxygen concentration is used as a measure of the oxygen storage capability and consequently of the efficiency and converting capability of the catalytic converter, in particular, the smaller the difference between the maximum value and the minimum value, the higher the efficiency of the catalytic converter.
Published Non-Prosecuted German Patent Application DE 196 30 940 A1 and the corresponding U.S. Pat. No. 5,862,661 disclose a further method of monitoring the functional capability of a catalytic converter, in which the mean value of the oxygen concentration downstream of the catalytic converter is determined from the signal of an oxygen sensor. The area bounded by the oscillation of the oxygen sensor signal about this mean value is used as a measure of the oxygen storage capability of the catalytic converter.
Both methods work in a continuous manner in a certain time pattern and consequently utilize the fact that the signal of the oxygen sensor downstream of the catalytic converter allows to ascertain the oxygen storage capability of the catalytic converter.
Both methods, in particular the method according to DE 196 30 940 A1, allow a relatively accurate ascertainment of the efficiency of the catalytic converter, but require essentially steady-state operating conditions of the internal-combustion engine, since the temperature of the exhaust gas of a lambda-controlled internal-combustion engine, and consequently the temperature of the catalytic converter, depends strongly on the operating condition of the internal-combustion engine. The non-isothermal behavior of the catalytic converter after non-steady-state operating conditions is particularly pronounced if the catalytic converter exhibits strong temperature fluctuations on account of the installation conditions. This is the case, for example, with a metal catalytic converter, which has a high thermal conductivity due to its physical properties. Catalytic converters which are provided close to the exhaust manifold and have a small volume are also subjected to strong temperature fluctuations. Such catalytic converters are increasingly used in internal-combustion engines, since they can be installed advantageously on account of the close proximity to the internal-combustion engine and, what is more, respond rapidly when the internal-combustion engine is started.
With the requirement for steady-state operating conditions, it is attempted to ensure that the catalytic converter is isothermally at the temperature of the exhaust gas. Then the temperature of the exhaust gas is taken into account when determining the efficiency of the catalytic converter.